The invention relates to return loss reduction for optical devices.
Optical devices often provide a certain amount of back reflection towards a source of an incident beam thus leading to so-called return loss usually defined as the ratio of the back reflected to the incident beam power. Optical attenuators, as a typical optical device, are used for attenuating the intensity of a light beam e.g. emerging from an optical fiber or an optical source to an output fiber. The attenuator comprises an attenuating device usually reflecting a portion of the incident beam and transmitting a portion of the incident beam as an output beam of the attenuator. The partly reflection of the incident beam generally leads to undesired back reflection and thus causes return loss. Tilting the attenuating device by a considerable angle generally leads to undesired polarization dependent loss and increasing the distances between attenuating element and emitting fiber disables building small instruments. A further effect resulting from back reflection might also be local heating due to absorption of the back reflected beam.
U.S. Pat. No. 5,546,212 discloses an optical module for two-way transmission. A semiconductor laser module is described in EP-A-595449. U.S. Pat. No. 6,137,941 shows a variable optical attenuator.
It is an object of the invention to provide an improved return loss of optical devices. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to the invention the emitted light beam of an emitting source is reflected at least partly by a partly-reflective element, which is tilted by a small angle to the optical axes of the input light beam for reflecting a portion of the emitted light beam towards a transmittive element in close proximity to the emitting source. Thus the return loss is reduced or eliminated and the polarization dependent loss is also optimized without increasing the size of the instrument. The invention allows a controlled guiding of the reflected portion of the emitted light beam in any desired direction and thus further allows a high power application.
In a preferred embodiment the partly-reflective element is tilted by a small angle to the optical axes of the input light beam so that the beam centroid of the reflected portion of the emitted light beam has a small lateral offset to the beam centroid of the emitted light beam and is directed to a transmittive element in close proximity to the emitting source, which is adapted to receive and transmit the reflected beam, so that substantially no portion of the received reflected portion is reflected towards the emitting source.
In a further preferred embodiment a mapping element is positioned between the partly reflective element and the transmittive element. The reflected portion of the emitted light beam is mapped in a controlled manner to the transmittive device causing a nearly complete transmission of the reflected portion away from the emitting source.
Advantageously by adjusting the tilt of the partly-reflective element the lateral offset can be determined. The tilt of the partly-reflective device is preferable limited for guiding the beam centroid of the reflected portion through the mapping element. The mapping element is preferably transmitting the reflected portion in opposite direction of the emitted input light beam.
Advantageously the mapping element comprises the collimating lens. This enables to collimate the emitted light beam transmitted by the partly-reflective element for coupling into an output fiber and also enables to map the reflected portion of the emitted light beam to the transmittive element.
In a further preferred embodiment the transmittive element is arranged for at least partly fixing or positioning an optical fiber, which is emitting the input light beam, and provided for transmitting the laterally offset reflected portion of the input light beam away from the input fiber.
The input light beam is according to a further preferred embodiment emitted by an optical fiber, which is at least partly fixed or positioned by a supporting device. This supporting device is provided for transmitting the thereto reflected portion of the input light beam with lateral offset from the focusing element. Thus, the supporting device transmits the reflected portion away from the fiber.
For transmitting away the reflected portion of the emitted light beam the transmittive element comprises in a preferred embodiment at least one optical fiber. This leads to a controlled transmission of the reflected portion away from the emitting source in any desired direction.
In a further preferred embodiment the transmittive element is housed in a ferrule element. The transmittive zone receiving the reflected portion is preferably positioned with lateral offset of the emitting source and enables to transmit all of the reflected portion.
In a further preferred embodiment the partly-reflective element is tilted in an angle smaller than 15xc2x0 with respect to the emitted light beam for preferably guiding the reflected portion through the mapping element and to reduce absorption of the back reflected beam.
The partly reflecting device in any of the inventive embodiments can be, for example, an optical attenuator etc.